What is Metal Oxide Varistor ?

MOV is one of the most widely used voltage limiting devices. Varistors include silicon carbide varistors and Zinc Oxide varistors.

The common use MOV is Zinc Oxide (ZnO) , it is a kind of over-voltage protect component , major material is zinc oxide which adding a variety of trace metal oxides, then mixed and molding, sintering and assembly . Its outside package with epoxy resin .

The varistor is a kind of voltage limiting protection device. Using the nonlinear characteristics of the varistor, when the overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value , then to protect the circuit . The main parameters of varistors include: varistor voltage, typical capacitance , Peak current , clamping voltage , surge current , response time, etc.

The response time of the varistor is ns (nanosecond) grade , it is faster than gas discharge tube and a little slower than TVS component . Generally, the response speed of overvoltage protection for circuits can meet the requirements. The order of magnitude range of MOV capacitance is in few hundred to several thousand pF .In many cases, it is not suitable to be directly applied to the protection of high frequency signal lines, when applied to the protection of AC circuits.Because its junction capacitance is a quite big and it will increase the leakage current , therefore it need to take full consideration when design the protection circuit . The impulse current of Metal Oxide Varistor is quite big , but its smaller than GDT .

The function of the MOV

It is equivalent to a variable varistor , which is connected to the circuit in parallel. When the circuit is in normal use, the impedance of the varistor is very high, and the leakage current is very small. It can be regarded as an open circuit and has little effect on the circuit. But when a very high sudden wave voltage comes, the resistance value of the varistor drops instantaneously , making it possible to pass a large current and clamp the overvoltage to a certain value. Because the surge resistance of varistors depends on its physical size, it is possible to obtain different surge current values.

One of the biggest characteristic of the MOV is that when the adding voltage lower than its threshold value “UN”, The current flowing through it is very small. like a closed valve . When the voltage exceeds “UN”, its resistance is smaller, so that the flowing current increase and not have much effect to other circuit , therefore reduce the overvoltage influence to the sensitive circuit . By using this function, the abnormal overvoltage often occurs in the circuit can be suppressed, and the protection circuit can be protected from overvoltage damage.

The Classification of varistor

The packaging forms of varistors are mainly two categories: disc shape and square shape.There are also have SMD ( surface mount device )type , large current modules and high voltage modules.The disc-shaped diameter :Ф5、Ф7、Ф10、Ф14、Ф20、Ф25、Ф32、Ф40、Ф53mm ( after packaging the produced product diameter will be a little big )The main part of the square shape is 34x34mm (S34 series)

The application of the MOV    

Small power protection machine; switch; POS; lightning arrester; building intercom; monitoring system; card parks - one card solution; transmission system; instrument; meter; communication product; control board .

The advantages and disadvantages of the varistor

Advantages :

1. The surge current is high (100A~70KA) ,The larger the volume, the larger the surge current is.

2. Full range type , widely use .

Disvantages:

1. The  the varistor nonlinear characteristic is quite poor . (The dynamic resistance is big )

2. High current limited voltage (clamp voltage) is higher

3. The leakage current is large at low voltage condition , and it is easier to age.

Selection of varistor

DC (working voltage) >1.5*AC (working voltage in the circuit)

Varistors should work within their specified parameter conditions, otherwise they may cause the deterioration or even breakdown .The failure mode of the varistor is mainly short circuit , If the short circuit time is too long, there will be an explosion, a fire, damage to the surrounding parts, and there may be happen an open circuit .

The following selection must be observed

The working environment of varistors should be within the technical specifications.

Ambient temperature :－40C～＋85 ℃

Relative humidity :in ＋40±2℃ condition , the maximum up to 96%   

Atmospheric pressure : Reach to 8.5KPa .

1. The AA supply voltage continuously applied to both ends of the varistor can not exceed the maximum continuous operating voltage ¡± value of the rated voltage listed ¡° in the specification table.We should take full consideration of the fluctuation amplitude of working voltage in the power network (or circuit) .

2. When selecting the varistor value of varistors, there should be have enough allowance.The general fluctuation range in China is 30%.The maximum surge current through the varistor should not exceed the maximum impulse current ¡± value in the technical specification ¡° .(That is the maximum impulse current) .Considering that it need tolerate multiple surge , the surge current should be able to withstand more than 10 times .

3. The clamping voltage of varistors must be smaller than the maximum voltage that can be sustained by protected parts or equipment (safety voltage)